add support CESU-8(utf8mb3) and MUTF-8(Java Modified UTF-8)

[Takehiko NOZAKI]

inspire from recent ruby’s change:

https://github.com/ruby/ruby/commit/09ff95c9751a56e4408a4809f0ed82c46729c8d5

also support Java Modified UTF-8(using alternate NUL as 0xc0 0x80).

[Takehiko NOZAKI]

done, see <<cset 88d176fd7035362fddce39143b82763a53c4fcf2>>.

[Takehiko NOZAKI]

oops, utf8mb3 != CESU-8, so should be dropped alias utf8mb3.

<<cset 46ae365d6f7fb9f15a2a84ed55ba4006491265e0>>